High frequency circuit



April 16, 1935. l. J; KAAR HIGH FREQUENCY CIRCUIT Filed April 29, 1935 Inventor: Ira -J. Kaar',

b M M His Attorney.

Patented Apr. 16, 1935 UNITED sTATEs HIGHFR'EQUENCY omoorr Ira J. Kaar, Schenectady, N. Y., assignor to General Electric Company, a, corporation of New York Application April 29, 1933, Serial No. 668,602

4 Claims.

My invention relates to high frequency circuits and more particularly to coupling circuits, or networks, for use between a high frequency source and a load.

In high frequency transmitting systems. it frequently happens that it is desirable. to supply any desired frequency within a wide range of frequencies from a transmitter to a single antenna, or to supply a single frequency froma transmitter. to antennae of widely different lengths and, of course, of widely different impedance characteristics. .At the same timeit is necessary for eflicient operation of the system that the circuits be so constructed and arranged that the transmitter works into a load of pure resistance of a certain optimum value dependent upon the characteristics of the dischargedevice employed and irrespective of. the impedance of the antenna at the particular frequency at which-the system operates.

One of the objects of my invention is toprovide improved means whereby this result is effected.

A further object of my invention is to provide means whereby a particular transmitter may be caused emciently to supply oscillations to antennae of somewhat greater range of lengths than has heretofore been possible.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof may best be understood by reference to the following descriptiontaken in connection with the accompanying drawing in which I have shown in Figs. 1, 6, and 7 certain embodiments of my invention; in Figs. 2 and 5 certain characteristics relating toits operation; and in Figs. 3 and 4 certain details.

Referring to Fig. 1 of thedrawing I have conventionally indicated therein an electron discharge device which may for example comprise the last stage of a high frequency transmitter. This transmitter has a tuned output circuit 2 comprising an inductance 3 and a pair of condensers 4. The upper terminal of the oscillatory circuit is connected to the anode of the discharge device and the lower terminal is connected to the cathode of the discharge device through a' condenser 53 having capacitance equal to the internal capacitance between the anode and cathode of the discharge device. As thus arranged the opposite sides of theoscillatory circuit 2 are capacitively balancedwith respect to ground and the discharge device I is thereby adapted to operate into a balanced load circuit,- such for example as an antenna of the doublet type, or into a balanced transmission line. It, will be understood, however, that this type of trans: mitter circuit isdescr-ibed simply by way of i1lus.- tration and that my invention is in no wise limited to systems adapted only for operation into balanced loads. H g g The impedance characteristics of the load cir- 1o; cuit into which the transmitter operatesrnay of course vary widely both in reactance and in resistance, whereas it is necessary to efficient op. eration of the discharge device I that it operate into a pure resistance load having a certain optimum value dependent upon the characteristics of. the electron discharge deviceemployed. To efiect this result inductances B, tuning inductances 1, and tuning condensers 8 are provided in each side ofthe output circuit together 20 with a portion of artificial line 9 which I shall hereinafter describe, and switching devices Hi and H. The purpose of the inductance 1 and condenser 8 is primarily to provide means whereby the reactance of the load circuit may be neu- 25 tralized. The inductance 6 is of such a value that in combination with the other elements of the system it causes the pure resistance of the load circuit when tuned by elements I and ii to be translated into a value suited forbest op- 30 eration of the discharge device.

To explain more fully the operation of the system shown in Fig. 1 reference may be made to Fig. 2 showing the impedance characteristics of antennae in general. The curve A of this fig- 35 ure shows the variations inthe reactance ofa vertical antenna with frequency. It will be observed that at low frequencies the reactance of the antenna is relatively large, but that it progressively decreases to zero at a point c cor- 40 responding to the natural frequency of the antenna. It then becomes positive and pro- I gressively increases to an extremely .highlvalue after which it suddenly drops through zero-to an extremely high negative value, after which it again progressively diminishes. This cycle repeats itself successively at the higher frequencies. The curve B represents the relation between the resistance of the antenna and the frequency. It will be observed that the resistance has maxi- 5 mum values at frequencies approximately mid+ way between points of the curve A corresponding to maximum positive and negative reactance, or where the reactance more abruptly changes sign. r I

It will be seen from these curves that both the reactance of the antenna and the resistance vary over an extreme range at different frequencies and that if the tube i is to operate into a pure resistance load of optimum value it is necessary that the reactance of the antenna, or load circuit, be neutralized at the operating frequency and that its resistance be translated to the optimum value determined by the characteristics of the discharge device.

The inductance l and condenser 8 are provided primarily to tune out the reactance of the antenna and the reactance 6 is provided primarily to provide the proper translation in the resistance of the load although in the final adjustment of the system, as will presently be seen, these functions are shared by both of these elements as well as by the oscillatory circuitZ.

When the switch It is in the lower position it will be observed that the inductance l on each side of the circuit is in series with the antenna and the condenser 8 is connected between the antenna and ground. The position of these two elements in the circuit is reversed when the switch I0 is in its upper position. By varying these two elements of the circuit the reactance of the antenna may be entirely balanced out over a certain range of frequencies.

The inductance 5 may be considered as one element of a network such as that shown in Fig. 3 in which is shown an inductance X2 connected in series with the line and a condenser X1 connected in shuntwith the line on the input side of the inductance X2. It can readily be shown that if such a network be connected between a source having a resistance R0 and a'load R and that if the values X2 and'X1 be adjusted as follows and 1 The capacitance of this condenser X1 is there- This I fore incorporated into the condensers means that the oscillatory circuit 2 is not, in itself, tuned to the frequency at which the system operates, but instead is tuned to that frequency only when considered in combination with the inductance 6 and the load, including the tuning means. The oscillatory circuit, of itself, is actually tuned to a frequency different from they frequency atwhich the system operates, the capacitance 4 being increased by an amount equal to the required capacity X1. The

load circuit then presents a unity power factorloadto the tube.

The impedance matching networkmay if desired be of the form shown in Fig. 4 in which the series element X2 is a capacitance and the shunt element X1 is an inductance. In this case the inductance 3 may be so constructed as to render the additional element X1 unnecessary, i. e., its inductance may be reduced by an amount equivalent tofthe addition of the inductance of element X1. Itis found, however, in practical application of the system that the form shown inFig. 3 is preferable owing to its inherent property of suppressing harmonic frequencies due both to its inductance and shunt capacitance.

The impedance matching network shown in both Figs. 3 and 4 are of the step-down type, the step-down being in the direction of the arrows, that is, the input resistance R0 is greater than the resistance R. If it happens, in a particular application, that the resistance of the antenna be greater than the optimum resistance into which the tube should operate, then these networks may be reversed in the circuit, or in other words, may be modified by connecting the reactance. X1 across the circuit at the opposite end of the reactance X2. In this latter case the reactance X1 may be provided in the case of Fig. 3 by so tuning the antenna that it is capacitive to an extent sufiicient 'to effect the desired impedance match, or in the case of Fig. 4, by so tuning the antenna that it is inductive by an amount sufficient to effect the impedance match.

In the methods above described the shunt element of the impedance matching network is either incorporated into the tuned circuit 2 or into the antenna tuning element as the case may require. These methods are satisfactory for operation over a wide range of frequencies on either side of the different points C of the curve A of Fig. 2. At frequencies beyond these ranges, however, a different adjustment of the apparatus may be necessary in order to permit the use of the same equipment at these frequencies. Such an adjustment may be necessary, for example, in case where a condenser, or inductance, which is adapted for tuning of the antenna to a frequency near one of the points C of the curve A of Fig. 2 may be unadapted for tuning to a frequency widely different from a frequency corresponding to these points ofthe curve or vice versa. In such a case it may be impossible entirely to tune out the reactance of the antenna by means of the antenna tuning elements 1, 8. Accordingly an impedance having both resistance and reactive components are presented to the output of the impedance matching network. The impedance looking into the input side of the impedance matching network will then, likewise, have both resistanceand reactive components. It has been found, however, that by proper adjustment of the series element 6 and by tuning the circuit 2 to a frequency slightly different from the frequency at which the system operates by an amount sufficient to neutralize the reactance of the impedance looking into the matching network, the desired unity power factor load of optimum value may be presented to thetube. plished even though neither the circuit 2 nor the antenna are tuned per se, to the operating frequency. In this case all of the elements of the system described cooperate to present a unity power factor load of optimum value to the tube.

By so adjusting the system the frequency range over which the same equipment may be employed is considerably extended. In certain extremely narrow ranges of frequency, however, as for ex ample in a range of frequencies of approximately 100 cycles where the reactance is extremely high, it is desirable that certain additional means he provided to accomplish the required tuning and impedance match. This means comprises in accordance with my invention, the additional line 9 which when included in the circuit actually changes the length of the antenna. This element of the system may comprise a conductor arranged in zigzag form on a non-conducting board in the transmitter housing, for example,

series This may he accomtance and inductance and to have distributed ca-.

pacitance to ground and distributed inductance substantially equal to that of an equivalent length of the actual antenna itself. Thus by opening the switches H and thereby including such a unit in circuit with the antenna the actual length of the antenna is changed to one at which tuning may be more readily effected. This is illustrated in Fig. 5 in which the curve D may be considered as the reactance curve of the antenna employed. If it were desired to tune the antenna sharply at a frequency closely approximating one of the frequencies at which the antenna has maximum reactance difiiculty might be encountered. This difficulty, however, may be obviated by opening the switch I l and thereby changing the antenna to one having an impedance characteristic as indicated by the curve E of Fig. 5. The frequencies referred to then, respectively, fall upon the curve E at the points F such that the impedance is one whereby tuning maybe readily effected.

The condensers l2 shown in Fig. 1 are blocking condensers to prevent the direct current electro motive force which is supplied to the anode of the discharge device from being impressed upon the remainder of the system.

A switch i3 is provided for the purpose of disconnecting one side of the line in cases where the transmitter is to be utilized to supply an antenna of the vertical type, for example, or a transmission line one side of which is grounded.

Fig. 6 shows an embodiment of my invention similar to Fig. 1 except that the switches i and H are removed. It is found in most practical cases that these elements are unnecessary. The tuning device comprises the series inductance l and shunt capacitance 8. This arrangement of the tuning elements is preferred since it has the effect of reducing harmonics. A switch 84 is provided to connect the system to a vertical antenna which is indicated at i5 and to disconnect it from a transmission line which may extend from the terminals it. When the switch M is in its upper position the additional switch it is moved to its right-hand position thereby short circui'ting the impedance elements 6, l, and 12 in the lower portion of the network. When the switch M is in its lower position the switch ll may be moved to the left for operation as described in connection with Fig. 1.

The arrangement shown in Fig. '7 differs from that shown in Figs. 1 and 6 in that the transmitter is indicated as one of the push pull type adapted for operation into a balanced transmission line or antenna of the doublet type. The system 18 comprises the tuning network and cooperates with the impedance matching element 6 and oscillator circuit 2 in the same manner as has been described in connection with Fig. 1.

While I have shown particular embodiments of my invention it will be understood that I do not wish to be limited thereto sincemany modifications may be made both in the circuit arrangement and instrumentalities employed and I contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent in the United States is:

1. In combination, an electron discharge device having an oscillatory output circuit, a load connected thereto having reactance, tuning elements for both said oscillatory output circuit and said load, the reactance of said load being such that said tuning element for said load is incapable of tuning said load to resonance at the frequency supplied by said electron discharge device, a reactive element between said output circuit and load, the tuning element for said oscillatory output circuit being so adjusted relative to the reactance of said element that said oscillatory output circuit is sufiiciently detuned from the frequency at which said discharge device operates to cause said discharge device to operate into a load of pure resistance and of optimum value.

2. In combination, an electron discharge dethe reactance of said element, and the reactance of said output circuit being so proportioned with respect to each other that said electron discharge device operates into a load of pure resistance of optimum value at the frequencies at which the reactance of said antenna is not completely neutralized by said means.

3. In combination, an electron discharge device, arranged for operation at a certain frequency, an oscillatory output circuit therefor, an antenna circuit, and a reactive element connected between said oscillatory output circuit and said antenna circuit, one of said circuits being tuned to resonance at a frequency different from the frequency at which said electron discharge device operates and so related to the reactance of said element that said discharge device operates into a load of pure resistance and of optimum value.

l. In combination. an antenna, means to supply high frequency oscillations thereto, said antenna having such extremely high reactance at the frequency supplied by said means as to render impractical the tuning of said antenna by concentrated reactance, and means to reduce said reactance, said means comprising a non-radiating conductor connected in series with said antenna, said conductor having distributed inductance and capacity approximately equal to that of an equal length of said antenna, and having length such that the combined reactance of said antenna and conductor is substantially lower than that of said antenna.

IRA J. KAAR. 

